Method for scroll-free machining rottionally symmetrical surfaces

ABSTRACT

A process for cutting machining of rotating rotationally symmetrical workpieces with a defined cutting edge, wherein the resulting rotationally symmetrical surface has either no twist or a twist which is only insignificant in terms of transport effect and abrasive action and/or when machining the hardened workpiece surfaces a particularly high material removal rate can be achieved, wherein a cutting edge which is askew with respect to the axis of rotation of the workpiece is guided in contacting relationship along the rotating workpiece in a linear feed movement transversely with respect to the axis of rotation, wherein the machining parameters, in particular the broaching feed and the inclined positioning of the cutting edge are so selected that the twist occurring at the machined surface becomes a minimum in respect of twist pitch and/or twist depth and in particular the twist depth becomes a minimum.

I. FIELD OF USE

[0001] The invention concerns the cutting machining of rotating, inparticular concentric rotationally symmetrical surfaces of a metalworkpiece, in particular of steel or cast iron, even in the hardenedstate, by means of a geometrically defined cutting edge or cuttingedges.

II. TECHNICAL BACKGROUND

[0002] The rotary machining of rotationally symmetrical parts both inthe soft and also in the hardened state with a geometrically definedcutting edge is state of the art. The processes involving grinding,finishing, honing and similar processes are in part substituted in thatrespect.

[0003] Cutting materials which have a sufficient service life, even formachining hardened workpieces, are available in the meantime in variousdifferent forms.

[0004] Machining after the hardening operation is generally necessary,even if the preliminary machining operation is effected with a highdegree of precision, as, after the hardening process, in partconsiderable degrees of distortion due to hardening generally occur. Aprocess which restores the dimensional accuracy of the workpiece againis therefore essential.

[0005] Longitudinal turning always produces a surface which suffers froma spiral rifling twist thereon, more particularly, irrespective of whichmaterial is machined and whether it is hardened or unhardened.

[0006] That surface has regular structures (grooves or burr-shapedraised portions) which correspond to a screwthread structure (twist) andwhich, by virtue of the feed movement of the tool along the rotatingworkpiece, produce a pitch.

[0007] That applies equally to workpiece surfaces which are cylindrical,conical or of some other shape. Therefore the surface to be produced onthe workpiece involves for example shapes of a screwthread or portionsthereof.

[0008] The formula relationship which describes the screwthread depth orroughness of the thread-like structure is described in FIG. 3.

[0009] As the cutting machining data, particularly when dealing withhardened workpieces, for high levels of surface quality, are generallyvery small, this frequently involves a relatively slow machining advanceor a low level of machining output.

[0010] While the relatively low level of machining output involves amarked economic disadvantage, the twist-bearing surfaces give rise toproblems in relation to seals which bear thereagainst, particularly ifthey are moving relative to the surface, for example a rotating shaftwithin a stationary seal. Seals in that sense can be for example theknown radial shaft sealing rings.

[0011] A surface which involves a twist configuration thereon in thatfashion, by means of the thread-shaped grooves or burrs, conveyscoolant, lubricant etc along the surface and past the seal bearingthereagainst, from one side to the other in the axial direction, so thatthe sealing action of the seal is considerably reduced. Particularly inthe case of machines which for example for reasons of hygiene or alsofor reasons of environmental protection, must run without leakage, thisrepresents a problem which is to be taken seriously.

[0012] In addition, the sealing elements which bear against the surfacewith the twist configuration thereon suffer a great deal of damage withtime at the contact lines or surfaces therebetween, due to thosethread-shaped grooves or raised portions, or they are at least subjectedto a severe abrasive effect. Due to that wear or damage, the sealingaction is also often seriously reduced or eliminated after a shortperiod of time.

[0013] If consideration is given to the available cutting machiningprocesses on a rotating workpiece, having regard to those twoproblematical aspects, the picture which results is as follows:

[0014] If, when turning rotationally symmetrical surfaces, the attemptis made to avoid the thread-shaped configuration being formed thereon bythe tool being moved relative to the workpiece only radially (plunge-cutturning), then a twist-free surface is produced, because of the absenceof any axial movement. If however the plunge-cut cutting edge is as widein the axial direction as the rotationally symmetrical surface which isto be produced overall, very high cutting forces occur, particularlywhen machining hardened surfaces, and there is a high tendency tochatter because of dynamic instability. Such dynamic instabilities orchatter result almost all of a sudden in such severe surfaceirregularities that here the surface is also too irregular to affordsatisfactory sealing integrity.

[0015] If the plunge-cut cutter is additionally moved lengthwise, thatis to say in the axial direction, when dealing with relatively widesurfaces to be produced, then it will be appreciated that a surface witha twist configuration thereon is again produced.

[0016] It is therefore state of the art and necessary that the twistproduced, in particular the thread-shaped turning grooves or tool markshave to be sufficiently reduced or even entirely eliminated withsubsequent expensive additional procedures, in order thereby to ensure asatisfactory sealing action.

[0017] One possible way of avoiding the twist configuration on thesurface (turning grooves or tool marks) could be the turning broachingprocess, wherein the broaching tool is moved in the tangential directionpast the rotating workpiece. If the individual cutting edges of theturning broaching tool are oriented parallel to the axis of rotation ofthe tool, this procedure again involves the problems of the high forcesacting on the cutting edge, and thus the trend towards dynamicinstability and the tendency to chatter.

[0018] If the cutting edge is positioned inclinedly in the turningbroaching operation, which results in a reduction in the pressure on thecutting edge however, that gives rise to a thread-shaped residualstructure as the points of engagement of the cutting edge on theworkpiece are at different spacings (radii) from the axis of rotation ofthe workpiece.

[0019] In a turning broaching operation by means of a disc-shaped toolwith the cutting edges on the periphery of the tool, inclinedpositioning of the cutting edge which in itself is straight also givesrise to the problem that in that case the surface produced on theworkpiece is a spherically convex surface instead of being an exactlycylindrical surface.

[0020] In addition in many cases grinding of the surfaces is applied asan additional procedure. That means that the workpiece generally has tobe transposed to another type of machine. The item costs of theworkpiece are therefore considerably increased by virtue of the increasein the length of the process chain, that is to say by the use of afurther machine, and the economic result is thus significantly worsened.In addition, in regard to final machining of the workpieces, if possiblegrinding should be avoided as this generally takes place in the form ofa wet process and thus further problems in terms of environment anddisposal occur in regard to the grinding slurry and in accordance withthe present state place a further burden on the economic result.

[0021] Added to that is the fact that even in the grinding operationtwist structures are produced, which are firstly produced on thegrinding disc by the dressing operation and which finally are reproducedon the workpiece. A finishing procedure in which a grinding band or agrinding element is applied to the workpiece also involves the formationof surface structures involving a twist configuration, because of theadditional oscillation or longitudinal movement of the finishing toolrelative to the workpiece.

III. STATEMENT OF THE INVENTION

[0022] a) Technical Object

[0023] Therefore the object in accordance with the invention is toprovide a process for machining rotating rotationally symmetricalworkpieces with a defined cutting edge, in which the resultingrotationally symmetrical surface has either no twist or a twist which isonly insignificant in terms of a transport effect and an abrasive actionand/or a particularly high metal removal rate can be achieved whenmachining the hardened workpiece surfaces.

[0024] b) Attainment of the Object

[0025] That object is attained by the features of claim 1. Advantageousembodiment are set forth in the appendant claims.

[0026] In accordance with the invention it has been found that theoccurrence of and/or the extent of the screwthread shape on the surfacestructure produced can be so influenced by the magnitude of the feedspeed of the cutting edges in relation to the existing inclinedpositioning of the cutting edge with respect to the axis of rotation ofthe workpiece, the predetermined diameter and the rotary speed of theworkpiece to be machined, that no twist or a twist which is no longerrelevant in a practical context occurs on the machined surface.

[0027] A further advantage is that this specific manner in accordancewith the invention of implementing broaching machining can be carriedout very much more quickly and at lower cost than for example agrinding, finishing or honing machining operation (or a comparableprocess) for removing the twist structure from the surface.

[0028] Thus, the turning broaching process according to the inventioncan be carried out on a normal turning machine or lathe, as well as on aturning broaching machine with for example a disc-shaped main tool body,if the cutting edge used for the process according to the invention canbe moved linearly and transversely, in particular perpendicularly, withrespect to the direction of the workpiece, in particular in a tangentialdirection, past the rotationally symmetrical surface of the workpiece,which is to be machined.

[0029] If—depending on the respective use of the workpiece—the aspect ofminimising the twist or freedom from twist on the surfaces to beproduced is not to the fore, the machining parameters can be so alteredthat the surface produced admittedly suffers from a twist thereon, butthe metal removal rate can be considerably increased, which is ofsignificance particularly when machining hardened workpieces. Thataffords inter alia a highly economic process with high levels of surfacequality.

[0030] c) Embodiments

[0031] An embodiment in accordance with the invention is described ingreater detail by way of example hereinafter with reference to theFigures in which:

[0032]FIG. 1 is a perspective view of the machining situation,

[0033]FIG. 2 is a view of the situation in FIG. 1 transversely withrespect to the direction of rotation 10 and to the feed movement 3, and

[0034]FIG. 3 is a view of the situation when implementing longitudinalturning.

[0035] First of all FIG. 3 shows how for example when implementinglongitudinal turning a thread-like surface is afforded by the cuttingtool which moves in the direction of rotation of the workpiece, and itscutting rounded configuration r_(E).

[0036] With a uniform advance feed f, expressed in mm/revolution of theworkpiece, the result is a helical groove 5, the pitch spacing of whichin the direction of the axis of rotation 10 is constant, with a uniformfeed f. In this respect, it will be seen that the depth t′ of thosegrooves depends on the size of the corner radius r_(E) of the cuttingtool producing the shape: the greater that corner r_(E) is, thecorrespondingly shallower are the flanks of the groove and thus thedepth t′ becomes correspondingly less. The relationship in terms of aformula reads as follows: $t = \frac{f^{2}}{8 \cdot r_{E}}$

[0037] In this respect, setting at least one of the edges of the cutter,in particular the secondary cutter, which lead to the cutter corner, atthe smallest possible angle, preferably parallel, with respect to thedirection of the axis of rotation 10, generally involves the only viableoption for minimising the depth t′ as, by virtue of predetermined cycletimes etc in production the feed f cannot be reduced just as may bedesired.

[0038] In comparison, FIGS. 1 and 2 show the situation according to theinvention:

[0039] As FIG. 1 shows, the workpiece 1 on which the rotationallysymmetrical surface 1 a to be machined is disposed rotates about theaxis of rotation 10, in the case of turning machines or lathes orturning broaching machines usually referred to as the Z-direction, withthe X-direction and the Y-direction usually each being perpendicularthereto.

[0040] The rotationally symmetrical surface 1 a to be machined is of awidth b in the Z-direction and is generally intended to serve as asupport surface for a radial shaft sealing ring or comparable element insubsequent use.

[0041] The tool 2 is guided along the workpiece, with its cutting edge 2a which is disposed inclinedly with respect to the Z-direction, in afeed direction 3 which is parallel to one of the tangential surfaces atthe rotationally symmetrical surface 1 a, transversely with respect tothe direction of the axis of rotation 10, in such a way that theindividual cutting regions successively come into contact with differentregions in the Z-direction of the rotationally symmetrical surface 1 a,in which respect to the direction extent of the cutting edge 2 a in thedirection of the axis of rotation, that is to say in the direction ofrotation 10, is equal to or greater than the width b of the surface a tobe machined. In this case, the feed direction of the tool can be in aplane which is perpendicular to the direction of rotation 10 of theworkpiece, that is to say in a radial plane of the workpiece, or in aplane which is disposed inclinedly relative to the direction of rotation10 (feed direction 3′).

[0042] When considered in the direction of rotation 10 of the workpiecethe feed direction 3 or 3′ however (see Figure 1a) always represents atangent to the reference or target diameter of the rotationallysymmetrical surface to be machined.

[0043] During the machining operation the workpiece 1 rotates in aturning direction 7 in such a way that, in the contact region betweenthe cutting edge 2 a and the surface 1 a to be machined, the surface ofthe workpiece 1 runs against the cutting edge 2 a.

[0044] The view in the Y-direction in FIG. 2 shows the inclinedpositioning angle α with respect to the direction of rotation 10 of theworkpiece 1 and the feed movement 3 of the cutting edge 2 a, which inthe view is at a right angle to that direction of rotation.

[0045] The thread-like surface structure shown in FIG. 2 is produced asa result of the peeling or pairing movement which takes place from theright-hand corner to the left-hand corner of the cutting edge 2 a inFIG. 2 in the machining procedure. That surface structure is formed by athread-like groove 5, the individual turns of which are in directlyadjoining relationship in the longitudinal direction of the workpiece,and form between them a thread burr 6.

[0046] The grooves 5 are shown in greatly exaggerated form in comparisonwith reality. The depth of the groove 5 is referred to as the twistdepth t while the spacing measured in the direction of rotation 10between two turns of the burr 6 or the grooves 5 is referred to as thetwist pitch s.

[0047] The cutting edge 2 a can also be moved with respect to theworkpiece 1 along an advance or feed movement 3′ which is transversewith respect to the axis of rotation 10 and in particular perpendicularto the cutting edge 2 a, in which case then a somewhat smaller width ofthe cutting edge 2 a is required.

[0048] The twist pitch s depends on the broaching feed f_(r) which givesthe advance in the feed direction 3, measured in millimetres perrevolution of the workpiece, and on the inclined positioning angle α, asfollows: $s = \frac{f_{r}}{\tan \quad \alpha}$

[0049] The twist depth t depends on the radius (core diameter) asmeasured to the troughs between the burrs 6 of the workpiece at thesurface 1 a to be machined, and the broaching feed f, as follows:$t = {{- r} + \sqrt{r^{2}} + \frac{f_{r}^{2}}{4}}$

[0050] It will be clear from the view in FIG. 2 that, upon reaching atwist depth t=0 or at least t<1 μm, in particular t<0.4 μm, inparticular t<0.2 μm, the magnitude of the twist pitch s no longer playsany part.

[0051] The primary intention therefore is to keep the broaching feedf_(r) as low as possible, and in particular so low that the twist deptht becomes substantially smaller, in particular at least by a factor of3, and better by a factor of 5, than the roughness depth R_(z) of thesurface produced in that way. The roughness depth R_(z) is usually inthe range of between 1.5 μm and 6 μm. LIST OF REFERENCES  1 workpiece 1a rotationally symmetrical surface  2 tool  2a cutting edge  3, 3′feed movement  4 tangent  5 groove  6 burr  7 direction of rotation  8working line 10 Z-direction, axis of rotation 11 X-direction 12Y-direction f feed f_(r) broaching feed α inclined positioning angle rworkpiece radius s twist pitch t twist depth b width of the surface 1aR_(z) roughness depth

1. A process for cutting machining of rotationally symmetrical surfaces(1 a) with a rotating workpiece (1), characterised in that a cuttingedge (2 a) which is askew with respect to the axis of rotation (10) ofthe workpiece (1) is guided in contacting relationship along therotating workpiece (1) in a linear feed movement (3, 3′) transverselywith respect to the axis of rotation (10), wherein the machiningparameters, in particular the broaching feed (f_(r)) and the inclinedpositioning (α) of the cutting edge are so selected that the twistoccurring at the machined surface (1 a) becomes a minimum in respect oftwist pitch (s) and/or twist depth (t) and in particular the twist depth(t) becomes a minimum.
 2. A process for cutting machining ofrotationally symmetrical surfaces (1 a) with a rotating workpiece (1),characterised in that a cutting edge (2 a) which is askew with respectto the axis of rotation (10) of the workpiece (1) is guided incontacting relationship along the rotating workpiece (1) in a linearfeed movement (3, 3′) transversely with respect to the axis of rotation(10), wherein the machining parameters, in particular the broaching feed(f), when machining a hardened rotationally symmetrical surface (1 a),are so selected that the resulting material removal rate becomes amaximum.
 3. A process according to claim 1 or claim 2 characterised inthat the linear feed movement (3, 3′) takes place in a tangential planeof the rotationally symmetrical surface (1 a).
 4. A process according toone of the preceding claims characterised in that the cutting edge (2 a)is a cutting edge (2 a) which is straight in itself.
 5. A processaccording to one of the preceding claims characterised in that theinclined positioning of the cutting edge (2 a) with respect to the axisof rotation (10) is between 0° and 90°, in particular between 20° and50°.
 6. A process according to one of the preceding claims characterisedin that the broaching feed (f_(r)) relative to the inclined positioning(α) of the cutting edge, and the diameter of the rotationallysymmetrical surface (1 a) to be machined, is so selected that thegrooves (5) produced on the surface of the workpiece by the machiningoperation, in terms of twist pitch (s) and twist depth (t), have noscrewthread effect or a negligibly slight screwthread effect.